The archive contains one subdirectory with a name of the form bard-mozrepl-abcdefg. Create a new zip archive with the files in the subdirectory bard-mozrepl-abcdefg. This new archive must have the files at the root entry, not in a subdirectory.

The file layout should look like this:

chrome/ components/ defaults/ chrome.manifest install.rdf logo.png

Rename your new zip archive from .zip to .xpi. Launch Firefox Install the freshly created .xpi file into Firefox. For example you can drag and drop the file into Firefox. Start mozrepl in Firefox by going to the menu: